The present invention relates generally to magnetoresistive read sensors for use in magnetic read heads. In particular, the present invention relates to the simultaneous fixation of the magnetization direction in pinned layers of first and second spin valves of a dual giant magnetoresistive read sensor.
A magnetic read head retrieves magnetically-encoded information that is stored on a magnetic medium or disc. The magnetic read head is typically formed of several layers that include atop shield, abottom shield, and aread sensor positioned between the top and bottom shields. The read sensor is generally a type of magnetoresistive sensor, such as a giant magnetoresistive (GMR) read sensor. The resistance of a GMR read sensor fluctuates in response to a magnetic field emanating from a magnetic medium when the GMR read sensor is used in a magnetic read head and positioned near the magnetic medium. By providing a sense current through the GMR read sensor, the resistance of the GMR read sensor can be measured and used by external circuitry to decipher the information stored on the magnetic medium.
A common GMR read sensor configuration is the GMR spin valve configuration in which the GMR read sensor is a multi-layered structure formed of a ferromagnetic free layer, a ferromagnetic pinned layer and a nonmagnetic spacer layer positioned between the free layer and the pinned layer. The magnetization direction of the pinned layer is fixed in a predetermined direction, generally normal to an air bearing surface of the GMR spin valve, while a magnetization direction of the free layer rotates freely in response to an external magnetic field. An easy axis of the free layer is generally set normal to the magnetization direction of the pinned layer. The resistance of the GMR read sensor varies as a finction of an angle formed between the magnetization direction of the free layer and the magnetization direction of the pinned layer. This multi-layered spin valve configuration allows for a more pronounced magnetoresistive effect than is possible with anisotropic magnetoresistive (AMR) read sensors, which generally consist of a single ferromagnetic layer.
Typically, the magnetization of the pinned layer is fixed in the predetermined direction by exchange coupling an antiferromagnetic layer to the pinned layer. The antiferromagnetic layer is positioned upon the pinned layer such that the antiferromagnetic layer and the free layer form distal edges of the GMR spin valve. The spin valve is then heated to a temperature greater than a Neel temperature of the antiferromagnetic layer. Next, a magnetic field oriented in the predetermined direction is applied to the spin valve, thereby causing the magnetization direction of the pinned layer to orient in the direction of the applied magnetic field. The magnetic field may be applied to the spin valve before the spin valve is heated to the temperature greater than the Neel temperature of the antiferromagnetic layer. While continuing to apply the magnetic field, the spin valve is cooled to a temperature lower than the Neel temperature of the antiferromagnetic layer. Once the magnetic field is removed from the spin valve, the magnetization direction of the pinned layer will remain fixed, as a result of the exchange with the antiferromagnetic layer, so long as the temperature of the spin valve remains lower than the Neel temperature of the antiferromagnetic layer.
A second GMR read sensor configuration is a dual GMR sensor having a first spin valve, a second spin valve, and a spacer positioned between the first and second spin valves. Both the first and second spin valves are formed of a free layer, a spacer layer, a pinned layer, and an antiferromagnetic layer. The spacer layer is positioned between the free layer and the pinned layer. The pinned layer is positioned between the free layer and the antiferromagnetic layer. The magnetization direction in the pinned layer of the first spin valve is antiparallel to the magnetization direction in the pinned layer of the second spin valve. The prior art method of fixing the magnetization directions in the pinned layers of the dual GMR sensor requires that the antiferromagnetic layers have substantially different Neel temperatures.
To fix the magnetization directions in the pinned layers of the dual GMR sensor, the multi-layered spin valve is first assembled. In the situation where the Neel temperature of the antiferromagnetic layer of the first spin valve is substantially greater than the Neel temperature of the antiferromagnetic layer of the second spin valve, the dual GMR sensor is heated to a temperature greater than the Neel temperature of the antiferromagnetic layer of the first spin valve. The GMR spin valve is then subjected to a first magnetic field oriented such that the magnetization direction in the pinned layer of the first spin valve orients in a desired direction. The first magnetic field maybe applied to the dual GMR sensor before the dual GMR sensor is heated. While continuing to apply the magnetic field, the dual GMR sensor is cooled to a temperature lower than the Neel temperature of the antiferromagnetic layer of the first spin valve, but greater than the Neel temperature of the antiferromagnetic layer of the second spin valve. The first magnetic field is next removed and a second magnetic field is applied to the dual GMR sensor. The second magnetic field is directed such that the magnetization direction in the pinned layer in the second spin valve orients in the desired direction, which is generally antiparallel to the desired direction of the magnetization direction in the pinned layer ofthe first spin valve. While continuing to apply the second magnetic field to the dual GMR sensor, the temperature of the dual GMR sensor is cooled to a temperature lower than the Neel temperature of the antiferromagnetic layer of the second spin valve.
The magnetization direction in the pinned layers of the first and second spin valve are now fixed, as a result of the exchange with the respective antiferromagnetic layers, so long as the temperature of the dual GMR sensor remains lower than the Neel temperatures of the antiferromagnetic layers. In the case where the Neel temperature of the antiferromagnetic layer of the second spin valve is greater than the Neel temperature of the antiferromagnetic layer of the first spin valve, the dual GMR sensor is first heated to a temperature greater than the Neel temperature of the antiferromagnetic layer of the second spin valve. The second magnetic field is then applied while the temperature ofthe dual GMR sensor is reduced to a temperature lower than the Neel temperature of the antiferromagnetic layer of the second spin valve, yet greater than the Neel temperature of the antiferromagnetic layer of the first spin valve. The second magnetic field is then removed and the first magnetic field applied while the temperature of the dual GMR sensor is reduced to a temperature less than the Neel temperature of the antiferromagnetic layer of the first spin valve.
The first and second spin valves can be connected in either a differential configuration or a gradiometer configuration. In a differential configuration, the output ofthe dual GMR sensor represents the difference between the voltage across the first spin valve and the voltage across the second spin valve. This differential configuration results in a read sensitivity greater than provided by a single spin valve GMR sensor. In a gradiometer configuration, the voltage measured across the first spin valve would be compared to the voltage measured across the second spin valve to measure the gradient of the magnetic field emanating from the magnetic media. This gradiometer configuration is useful in detecting peaks and valleys in the magnetic fields.
There are several inherent problems with the prior art method of fixing the magnetization direction of the pinned layers of a dual GMR sensor. First, the two antiferromagnetic layers must be annealed separately. For each layer, the annealing process can take hours, or even days. For a dual GMR sensor, this annealing process becomes twice as long as required for a single AMR sensor. Second, the temperature within an operating disc drive can reach fairly high temperatures. One of the antiferromagnetic layers of the dual GMR sensor has a Neel temperature substantially lower than the other antiferromagnetic layer. It is, therefore, more likely that the temperature within the operating disc drive would exceed the lower Neel temperature, causing the antiferromagnetic layer (and the pinned layer) associated with that spin valve to lose its fixed magnetization orientation. There is, therefore, a need for a better means of fixing the magnetization directions in pinned layers of a dual GMR sensor.